1. Field of the Invention
The present invention relates to a heat-sensitive flow rate sensor for use in measuring flow rate and flow velocity of a fluid and, more particularly, to a heat-sensitive flow rate sensor of the type which measures flow velocity and flow rate of a fluid-based on the rate at which heat is carried away from a probe by the fluid which flows in contact with the probe.
2. Description of the Related Art
In general, a heat-sensitive flow rate sensor is used as an intake air flow rate sensor which senses intake air flow rate in an automotive engine to provide air flow rate information to be used by an electronic fuel injector which controls the air-fuel ratio of a mixture to be fed to the engine. This type of sensor employs, as a flow velocity probe, a heat-sensitive resistor which generates heat by electrical power supplied thereto. The flow velocity of a fluid flowing in contact with this probe is measured based upon the rate of transfer of heat from this probe to the flowing fluid. Usually, a measuring circuit connected to the probe includes a bridge circuit and a differential amplifier which operate to control the electrical current supplied to the resistor so as to maintain the resistance of the resistor to a constant level, and the level of such electrical current is used as a signal indicative of the flow rate.
FIG. 6 schematically shows the construction of a known heat-sensitive flow rate sensor of the type which is disclosed in Japanese Patent Laid-Open No. 3-84425. A sensor tube 2, which forms a part of the fluid passage, is provided at a predetermined position in a housing 1 which defines a principal passage for the fluid. A flow velocity probe 3 including a heat-sensitive resistor as will be described later, as well as a fluid temperature sensor 4, is disposed at predetermined locations in the sensor tube 2. The flow velocity probe 3 and the fluid temperature sensor 4, together with resistors R1 and R2, form a bridge circuit. The junctions b and c of the bridge circuit are connected to a differential amplifier 101. The output of the differential amplifier 101 is connected to the base of a transistor 102. The transistor 102 is connected at its emitter to a junction a of the bridge circuit and at its collector to a power supply 103.
FIGS. 7A and 7B are a front elevational view and a side elevational sectional view of an example of the flow velocity probe 3 of the heat-sensitive flow rate sensor. Referring to these Figures, the flow velocity probe 3 has a substrate 5 made of an insulating material such as alumina. A heat-sensitive resistor 6 in the form of a film is provided on the substrate 5. The heat-sensitive resistor 6 is made of a material which varies its resistivity according to temperature. A patterning line wiring 7 is laid on the heat-sensitive resistor 6 so as to provide a path of electrical current. Lead lines 8 are connected to an end of the resistor 6. Each of the lead lines 8 is connected at its other end to the associated terminal 9 for connection to the electrical circuit. A protective coat 10 is formed on the heat-sensitive resistor 6 so as to protect the latter. The flow velocity probe 3 is held by a holder 11. The heat-sensitive resistor 6 generates heat over a vertical range thereof indicated by LH.
The operation of this known heat-sensitive flow rate sensor is as follows. When flow of a fluid at a constant flow rate exists in the housing 1, the bridge circuit is balanced in such a condition that the mean temperature of the heat-sensitive resistor 6 of the flow-velocity probe 3 is maintained at a level which is higher than the fluid temperature by a predetermined value. The mean temperature is maintained by controlling electrical current supply to the bridge circuit. This control is performed by a control circuit constituted by the differential amplifier 101 and the transistor 102. When the flow rate of the fluid increases, the cooling of the heat-sensitive resistor 6 is enhanced so as to cause a change in the resistivity of the resistor 6. As a result an imbalance is caused in the bridge circuit. The control circuit then operates to increase the electrical current supplied to the bridge circuit. Consequently, the heat-sensitive resistor 6 is heated so that the mean temperature of the resistor 6 is elevated to the level exhibited before the change in the fluid flow rate, whereby the bridge circuit is balanced again.
The heat generated by the heat-sensitive resistor 6 is not only carried away by the fluid but also is dissipated through the support substrate 5. Since the proportion of the heat dissipated through the substrate to the total heat generated by the heat-sensitive resistor 6 varies according to the flow velocity of the fluid, the temperature distribution over the holder 11 and the flow velocity probe 3 is varied according to the flow velocity. Therefore, when the flow rate of the fluid is drastically changed, the control circuit performs a transient control operation, thus failing to produce correct output, until a steady temperature distribution is recovered.
In order to overcome this problem, a proposal has been made in, for example, Japanese Patent Laid-Open NO. 2-264822, in which an additional heat generating body is provided in the vicinity of the boundary between the fixing portion of the sensor and the portion where the heat-sensitive resistor is formed. This proposal, however, inconveniently requires a flow velocity probe of a complicated construction and necessitates an operation for controlling the rate of heat generation from the additional heat generating body.
Thus, the known heat-sensitive flow rate sensor of the type described could not well respond to rapid change in the flow velocity. Improved sensor having an additional heat generating body undesirably requires complicated probe structure and control system and, hence, cannot be produced at low cost.